Gelatin has been used extensively in a variety of imaging elements as the binder because of its many unique and advantageous properties. For example, its property of water swellability allows processing chemistry to be carried out to form silver halide-based photographic images. However, due to this same property, imaging elements with exposed golatin-containing materials, no matter if they are formed on transparent or reflective media, have to be handled with care so as not to be in contact with any aqueous solutions that may damage the images. For example, accidental spillage of common household solutions such as coffee, punch, or even plain water can permanently damage photographic prints.
There have been attempts over the years to provide protective layers for gelatin-based photographic systems that will protect the images from damage by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A number of patents describe methods of solvent-coating a protective layer on the image after photographic processing is completed and are described, for example, in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. More recently, U.S. Pat. No. 5,376,434 describes a protective layer formed on a photographic print by coating and drying a latex on a gelatin-containing layer bearing an image. The latex comprises a resin having a glass transition temperature of from 30.degree. C. to 70.degree. C. Another type of protective coating involves the application of UV-polymerizable monomers and oligomers on a processed image followed by radiation exposure to form crosslinked protective layer, which is described in U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. A drawback for both the solvent coating method and for the radiation cure method is the health and environmental concern of those chemicals or radiation to the coating operator. Another drawback is that the photographic materials need to be coated after the processing step. Thus, the processing equipment needs to be modified and the personnel running the processing operation need to be trained to apply the protective coating.
Various lamination techniques are known and practiced in the trade. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating a polymeric sheet film, as a protective layer, on a processed image. However, protective coatings that need to be applied to the image after it is formed, several of which were mentioned above, add a significant cost to the final imaged product. A number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706.686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. More recently, U.S. Pat. No. 5,853,926 to Bohan et al. discloses a protective coating for a photographic element, involving the application of an aqueous coating comprising polymer particles and a soft polymer latex binder. This coating allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. Again, however, the hydrophobic polymer particles must be fused to form a protective coating that is continuous and water-impermeable.
The ability to provide the desired property of post-process water/stain resistance of an imaged photographic element, at the point of manufacture of the photographic element, and in a way that involves minimal or no changes in the photofinishing operation, is a highly desired feature. However, in order to accomplish this feature, the desired photographic element must be very permeable to aqueous solutions during the processing step, but become relatively water impermeable or water resistant after the processing is completed. Commonly assigned U.S. Ser. No. 09/235,436 discloses the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. Commonly assigned U.S. Ser. No. 09/235,437 and U.S. Ser. No. 09/448,213 disclose the use of a second polymer such as a soluble gelatin or polyvinyl alcohol to improve permeability.
U.S. Pat. No. 5,856,051 describes the use of hydrophobic particles with gelatin as the binder in an overcoat formulation. This invention demonstrated an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. The hydrophobic polymers exemplified in U.S. Pat. No. 5,856,051 include polyethylene having a melting temperature (Tm) of 55 to 200.degree. C., and therefore capable of forming a water-resistant layer by fusing the layer at a temperature higher than the Tm of the polymer after the sample has been processed to generate the image. The coating solution is aqueous and can be incorporated in the manufacturing coating operation without any equipment modification. The fusing step is simple and environmentally friendly to photofinishing laboratories. Since the particles are incorporated entirely within the uppermost layer, this approach does not suffer from a lack of mechanical strength and integrity during transport and handling prior to image formation and fusing. However, the scratch resistance of such an overcoat after fusing is a concern, since polyethylene is a very soft material. More durable materials cannot be used in this application because the crosslinked gelatin in the layer interferes with the film-formation process.
Similarly, commonly assigned U.S. Ser. No. 09/353,939 and U.S. Ser. No. 09/548,514, respectively, describe the use of a polystyrene-based material and a polyurethane-based material, with gelatin as the binder, in an overcoat for a photographic element, which overcoat can be fused into a water resistant overcoat after photographic processing is accomplished to generate an image. Like the polyethylene overcoats described above, the protective properties of this overcoat are compromised by the necessity to form a continuous film in the presence of gelatin in the layer. Only relatively low molecular weight polymers can be used, which afford protective overcoats with inferior properties. Further, the photofinishing operation must include a fusing step in order to achieve a protective layer.
U.S. Ser. No. 09/547,374 (Docket 80610) describes the use of a proteolytic enzyme incorporated into one of the processing solutions that removes gelatin from a nascent protective layer, which layer becomes water resistant upon drying. This method of providing a protective layer still requires modification of the photofinishing process, and is thus inconvenient for the photofinisher to implement.
Therefore, there remains a need for an overcoat applied to a photographic element before development that would not significantly reduce the rate of reaction of the developer with the underlying emulsions, that requires no substantial modification of the commercial photofinishing solutions, and minimal or no other modifications of the photofinishing operation, but that would ultimately provide a water resistant and durable overcoat after the processing or developing step. Furthermore, it would be desirable if the manufacture of the photographic element with the overcoat required no substantial modification of the manufacturing operation.